Bump debootstrap-image ubuntu_version to 20.04.1
19.10 reached EOL https://lists.ubuntu.com/archives/ubuntu-announce/2020-July/000258.html, so we should move to 20.04.
19.10 reached EOL https://lists.ubuntu.com/archives/ubuntu-announce/2020-July/000258.html, so we should move to 20.04.